Chemical vapor infiltration is a method used, in particular, in the fabrication of composite materials, and in particular composite materials having a matrix of carbon or of a ceramic. A fiber reinforcing substrate or "preform" having substantially the same shape as a composite material part to be fabricated is placed inside an enclosure. The substrate is densified by progressively filling its pores by injecting a flow of gas into the enclosure. Under determined conditions of temperature and pressure existing inside the enclosure, a solid deposit of matrix material is formed on the fibers of the substrate, right into the core of the substrate, by one of the components of the gas decomposing or by a reaction taking place between components of the gas in contact with the fibers. Chemical vapor infiltration is continued until a desired degree of substrate densification by the matrix is achieved.
Chemical vapor infiltration techniques for densifying a fiber substrate with a carbon matrix or with a ceramic matrix are well known. A carbon matrix is generally obtained by cracking alkanes, in particular methane or mixtures of methane and heavier alkanes. Chemical vapor infiltration of refractory materials other than carbon is described in Document FR-A-2 401 888. For example, a silicon carbide matrix can be obtained by decomposing methyltrichlorosilane (MTS) CH.sub.3 SiCl.sub.3 in the presence of hydrogen H.sub.2.
In order to form a deposit of the desired nature (composition and crystal structure) on the surface of the substrate fibers, it is necessary for the gas to be at the required temperature when it comes in contact with the substrate. Otherwise, undesirable secondary species may be deposited. Thus, in the above-outlined example of a silicon carbide matrix being formed by decomposing MTS, gas at too low a temperature can cause a deposit of silicon to be formed.
That is why proposals have already been made to bring the gas into contact with a preheater apparatus prior to diffusing it into the substrate. Thus, in the installation described in document FR-A-2 594 119, the flow of gas that penetrates into the enclosure comes initially into contact with superposed and perforated preheating plates. Since the preheating plates are inside the enclosure, they are permanently at the inside temperature of the enclosure, which temperature is determined to provide optimum infiltration conditions. The gas flow is thus raised to the desired temperature while it is passing through the preheating plates and prior to coming into contact with the substrates to be densified. The preheater plates also have the function of trapping parasitic deposits since these deposits tend to form on the surfaces first encountered by the gas flow on penetrating into the enclosure.
However, the preheating plates of that prior installation are made of graphite, and are therefore heavy and bulky. Because of the considerable mass of those plates, the time taken to heat the enclosure and to stabilize its internal temperature to the desired value is increased. In addition, because of their bulk, the preheating plates significantly reduce the working volume inside the enclosure. However, it is necessary to obtain optimum filling of the inside of the enclosure with the substrates to be densified because of the very high cost of fabricating composite material pans by chemical vapor infiltration. This cost is the result, in particular, of the extremely long duration (generally several hundreds of hours) required by the infiltration process to achieve the desired degree of substrate densification.
In addition, it has been observed that varying degrees of densification within pans or between different parts densified simultaneously are obtained in composite material parts fabricated by means of an installation such as that described in above-mentioned document FR-A-2594 119. The densification gradient seems to be caused, at least in pan, by non-uniformity of the gas in the working volume of the enclosure and/or by non-optimum flow conditions for said gas. It is desirable for the densification gradient within a part made of composite material to be reduced to as small a value as possible so as to obtain a matrix with as little anisotropy as possible, thereby improving the properties of the composite material. When a plurality of parts are densified simultaneously, it is desirable to avoid too great a dispersion in the characteristics of the resulting composite materials.
An object of the invention is thus to provide improved preheating apparatus capable of preheating a flow of gas penetrating into an enclosure of an installation for chemical vapor infiltration.
More particularly, an object of the invention is to provide preheating apparatus which is less heavy and bulky than the graphite plates used in the prior art, and which provides gas that is more homogeneous throughout the working volume of the enclosure.